In a motor vehicle having a power transmission powered by an automotive engine through fluid couplings or a torque converter, it is common to equip the vehicle with a parking brake mechanism which is capable of anchoring the output shaft of the power transmission when secure parking of the vehicle is required. However, conventional parking brake mechanisms of the type mentioned above have a bulky construction due to their complicated constructions requiring relatively big mounting spaces in the transmission. This means that the entire construction of the transmission becomes large.